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HUMMING-BIRDS 331classification, is generally in inverse proportion to its adaptabilityto special uses. And the reason of this is apparent,when we consider that similarities of food and habits are oftenaccompanied by similarities of external form or of specialorgans, in totally distinct animals. Porpoises, for example,are modified externally so as to resemble fishes ; yet they arereally mammalia. Some marsupials are carnivorous, and areso like true carnivora that it is only by minute peculiarities ofstructure that the skeleton of the one can be distinguishedfrom that of the other. Many of the hornbills and toucanshave the same general form, and resemble each other in habits,in food, and in their enormous bills ; yet peculiarities in thestructure of the feet, in the form of the breast-bone, in thecranium, and in the texture and arrangement of the plumage,show that they have no real affinity, the former approachingthe kingfishers, the latter the cuckoos. Such structuralpeculiarities as these have no direct relation to habits and;they are therefore little liable to change, when from any causea portion of the group may have been driven to adopt a newmode of life. Thus all the Old World apes, however muchthey may differ in size or habits, and whether we class themas baboons, monkeys, or anthropoids, have the same numberof teeth ;while the American monkeysall have an additionalpremolar tooth. This difference can have no relation to thehabits of the two groups, because each group exhibits differencesof habits greater than often occur between Americanand Asiatic species.; and it thus becomes a valuable characterindicating the radical distinctness of the two groups, a distinctnessconfirmed by other anatomical characters.On the other hand, peculiarities of organisation which seemspecially adapted to certain modes of life are often diminishedor altogether lost in a few species of the group, showing theiressential unimportance to the type,as well as their smallvalue for classification. Thus the woodpeckers are moststrikingly characterised by a very long and highly extensibletongue, with the muscles attached to the tongue-bone prolongedbackward over the head so as to enable the tongue to besuddenly darted out and also ;by the rigid and pointed tail,which is a great help in climbing up the vertical trunks oftrees. But in one group (the Picumni) the tail becomes quite